Systems and methods for forward osmosis fluid purification

ABSTRACT

A process for purification of fluids, for example, desalination of seawater or brackish water, using organic solutes in a concentrated water solution for use in a forward osmosis process, to extract fresh water out of salt water through the forward osmosis membrane, and subsequently separating the organic solutes out of the diluted forward osmosis permeate by cloud point extraction, thereby regenerating a concentrated organic solution for recycling to the forward osmosis process, and fresh water for potable water use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.12/338,990, filed Dec. 18, 2008, entitled SYSTEMS AND METHODS FORFORWARD OSMOSIS FLUID PURIFICATION, the entire contents of which areincorporated herein by this reference.

BACKGROUND

1. Technical Field

This disclosure relates generally to the purification of fluids and morespecifically to the desalination and purification of seawater orbrackish water.

2. Related Art

Today's world has increasingly greater need for fresh, drinkable water.Current desertification is taking place much faster worldwide thanhistorically, arising from the demands of increased populations. Inaddition, freshwater resources are being depleted, escalating the needfor a new economically efficient way to produce potable water.

The abundant availability of seawater and development of newtechnologies for desalination of brine and brackish waters will be apotential solution to the pressing worldwide problem of limitedfreshwater supplies and sources, with major societal benefits andimpacts. Thus there is global demand for desalination and fluidpurification in general.

Desalination using membrane processes, which mainly rely upon reverseosmosis (RO), are presently the fastest growing technology,approximating around 22% of world desalination capacity. Reverse osmosisuses dynamic pressure to overcome the osmotic pressure of saltsolutions, allowing water-selective permeation of salt-free water tomigrate from the saline side of a membrane to the freshwater side.However, RO systems need high pressures (50 to 100 atm or 800 to 1500psi) and extensive pre-treatment of seawater to allow sufficientpermeation through the RO membrane, leading to a seawater conversionrate between about 35 to 50%. Furthermore, initial membrane costs andreplacement issues lead to high capital investments and operating costsfor RO systems.

Notwithstanding the advancement in water purification techniques, and inparticular water desalination (purification) technology, the capital andoperating costs of these processes remain significantly higher than thecost of retrieving natural fresh water and delivering it as potablefresh water. Water, in the United States, is typically priced forwholesale at $100 to $450 per acre-foot, depending on geographiclocation and proximity to freshwater sources. In California, thewholesale cost of freshwater is approximately $500 per acre-foot.Presently, the cost of desalination is in the range of $617 to $987 peracre-foot of desalinated water. Desalination of brackish water by ROprocesses, while cheaper, is still relatively expensive, ranging from$247 to $432 per acre-foot.

Consequently, a need remains for water purification processes that canoperate with lower energy requirements, higher efficiency, and/or lowercosts than the current state-of-the-art fluid purification processes.

SUMMARY

The systems and methods disclosed herein utilize forward osmosis forfluid purification, such as seawater desalinization and brackish waterpurification. Various embodiments include the realization that thefunction of forward osmosis can be improved through the use of one ormore cloud-point solutes to generate high osmotic pressure differentialswith the salt water or fluid to be treated. The use of cloud pointsolutes can advantageously permit recycling of the forward osmosispermeate through cloud-point extraction. The result is systems andmethods that can operate with lower energy requirements, lower operatingtemperatures and pressures, higher efficiency, high flux rates offluids, and/or lower costs than previous fluid purification processessuch as reverse osmosis.

In certain embodiments, forward osmosis systems and methods aredisclosed comprising a closed processing loop. The systems and methodstake salt water input and discharge purified water for potable water useand concentrated brine for discharge to the ocean or to commercial uses.The cloud point solutes that drive forward osmosis of water from thesalt water are recovered through cloud point precipitation andfiltration and reused in future processing.

In various embodiments, forward osmosis fluid purification systems areprovided. A system can comprise a semi-permeable membrane having a feedsolution-facing surface opposite a draw solution-facing surface. Asystem can comprise a feed solution in communication with the feedsolution-facing surface of the semi-permeable membrane, wherein the feedsolution comprises a fluid to be purified and impurities dissolved inthe fluid to be purified. The system can comprise a draw solution incommunication with the draw solution-facing surface of thesemi-permeable membrane. The system can comprise a precipitation systemconfigured precipitate the at least one cloud point solute from the drawsolution.

A forward osmosis fluid purification system can comprise a feed solutionchamber and a feed solution disposed in the feed solution chambercomprising a fluid and impurities dissolved in the fluid. A system cancomprise a draw solution chamber and a draw solution disposed in thedraw solution chamber comprising the fluids and at least one cloud pointsolute dissolved in the fluid. A system can comprise a a semi-permeablemembrane disposed between the feed solution chamber and the drawsolution chamber separating the feed solution from the draw solution,wherein the membrane is configured to permit diffusion of the fluid fromthe feed solution into the draw solution. A system can comprise aprecipitation system in communication with the draw solution chamber,wherein the precipitation system is configured to cause precipitation ofthe at least one cloud point solute in the draw solution. A system cancomprise a a filtration system in communication with the precipitationsystem configured to separate at least a portion of the precipitated atleast one cloud point solute from the fluid in the draw solution. Asystem can comprise a return line in communication with the filtrationsystem configured to receive the separated at least one cloud pointsolute and return the at least one cloud point solute to the drawsolution chamber. A system can comprise an outlet line in communicationwith the filtration system configured to receive the fluid separatedfrom the draw solution.

The draw solution can comprise at least one cloud point solute. Theconcentration of the at least one cloud point solute in the drawsolution can be greater than the concentration of the impurities in thefeed solution. The feed solution can comprise seawater or brackishwater. The fluid to be purified can be water. The solubility of the atleast one cloud point solute in the fluid to be purified can be a molarratio of at least 3:1. The at least one cloud point solute can have amolecular weight between about 300 Da and 800 Da. The at least one cloudpoint solute can comprise a hydrophobic component and a hydrophiliccomponent. The at least one cloud point solute can comprise apolyoxyorganic chain. The at least one cloud point solute can comprise apolyethylene glycol or a polypropylene glycol. The at least one cloudpoint solute can comprise a polyethylene glycol selected from the groupconsisting of FA PEG-300, FA PEG-400, PEG-400 ML, and FA PEG-600. The atleast one cloud point solute can comprise an ethoxylate. The at leastone cloud point solute comprises a fatty acid ethoxylate or a fattyalcohol ethoxylate. The concentration of the at least one cloud pointsolute in the draw solution can be between about 30% and 75%. Theconcentration of the at least one cloud point solute in the drawsolution can be between about 50% and 70%.

The precipitation system can comprise a heater configured to heat thedraw solution to at least the cloud point temperature of the drawsolution. The precipitation system can comprises a gas treatment systemconfigured to add at least one water-insoluble gas to the draw solutionto lower the cloud point of the draw solution.

The system can comprise a filtration system configured to remove theprecipitated at least one cloud point solute from the fluid to bepurified in the draw solution. The filtration system can comprise ananofiltration membrane or an ultrafiltration membrane. The system cancomprise a redissolution system configured to redissolve theprecipitated cloud point solutes to form a recycled draw solution.

In various embodiments, methods for purifying fluids are provided. Amethod can comprise disposing a feed solution and a draw solutionopposite a semi-permeable membrane. The feed solution can comprise afluid to be purified and impurities dissolved in the fluid to bepurified. The draw solution can comprise at least one cloud pointsolute. The concentration of the at least one cloud point solute in thedraw solution can be greater than the initial concentration of theimpurities in the feed solution. The method can comprise allowing aquantity of the fluid to be purified to diffuse from the feed solution,through the semi-permeable membrane, and into the draw solution throughforward osmosis. The method can comprise precipitating the cloud pointsolutes from the fluid to be purified in the draw solution. The methodcan comprise treating the draw solution to cause precipitation of thecloud point solutes. The method can comprise passing the draw solutionthrough a filtration system to separate at least a portion of theprecipitated at least one cloud point solute from the fluid to bepurified. The method can comprise returning the separated at least onecloud point solute to the draw solution opposite the semi-permeablemembrane.

The feed solution can comprise seawater or brackish water. The fluid tobe purified can be water. Precipitating the cloud point solutes cancomprise adding at least one water-insoluble gas to lower the cloudpoint of the draw solution. Precipitating the cloud point solutes cancomprise heating the draw solution to at least the cloud point of thedraw solution.

The method can comprise removing the precipitated cloud point solutesfrom the draw solution. Removing the precipitated cloud point solutescan comprise filtering the draw solution through a nanofiltrationmembrane. The method can comprise redissolving the precipitated cloudpoint solutes to form a recycled draw solution. Redissolving theprecipitated cloud point solutes can comprise flushing thenanofiltration membrane at a temperature below the cloud pointtemperature of the cloud point solutes.

In various embodiments, assemblies for forward osmosis are provided. Anassembly can comprise a semi-permeable membrane having a feedsolution-facing surface opposite a draw solution-facing surface. Theassembly can comprise a feed solution input in communication with thefeed solution-facing surface of the semi-permeable membrane, wherein thefeed solution input is configured to supply a feed solution comprising afluid to be purified and impurities dissolved in the fluid. The assemblycan comprise a draw solution preparation in communication with the drawsolution-facing surface of the semi-permeable membrane, wherein the drawsolution preparation comprises at least one cloud point solute.

The at least one cloud point solute can have a molecular weight betweenabout 300 Da and 800 Da. The at least one cloud point solute cancomprises a hydrophobic component and a hydrophilic component. The atleast one cloud point solute can comprise a polyoxyorganic chain. The atleast one cloud point solute can comprise a polyethylene glycol or apolypropylene glycol. The at least one cloud point solute can comprise apolyethylene glycol selected from the group consisting of FA PEG-300, FAPEG-400, PEG-400 ML, and FA PEG-600. The at least one cloud point solutecan comprise an ethoxylate. The at least one cloud point solutecomprises a fatty acid ethoxylate or a fatty alcohol ethoxylate.

In various embodiments, processes for purification of solvents areprovided. A process can comprise inducing forward osmosis across asemi-permeable membrane by creating an osmotic pressure differentialusing a draw solution comprising one or more solutes having a cloudpoint below the boiling point of the solvent, whereby the draw solutionbecomes more diluted as solvent is drawn across the membrane. Theprocess can comprise heating the diluted draw solution to just above thecloud point temperature to trigger phase separation of the solute. Theprocess can comprise separating the solutes from the draw solution byfiltration. The process can comprise retrieving the solutes forrecyleable use in the forward osmosis process.

At least one of the solutes can comprise an organic compoundcharacterized by a high osmotic pressure. At least one of the solutescan comprise a polymeric compound characterized by a high osmoticpressure. The solute can comprise a molecular weight of about 200-600.

In various embodiments, processes for desalination of salinated waterare provided. A process can comprise osmotically separating pure waterfrom salinated water by inducing forward osmosis through asemi-permeable membrane by creating an osmotic pressure differentialacross the membrane using concentrated solutions of solutes in water tocreate a solution with higher osmotic pressure than the desalinatedwater. The osmotic separation can serve to drive pure water across themembrane from the salinated water side to the side containing thesolutes by virtue of the high solubility and high osmotic pressures inthe solvent of the solutes being used. The osmotic separation throughforward osmosis can result in a diluted solution. The process cancomprise inducing clouding of the diluted solution of solutes by heatingthe solution to above the cloud point of the solutes by reaching atemperature at which a solubility inversion of the solute in the solventoccurs, thereby reducing its solubility and causing it to precipitateout of the solution. The process can comprise separating potable wateras a permeate from the organic solutes by a mechanism of filtration. Theprocess can comprise re-dissolving the clouded solutes filtered out fromthe water by reverse flushing the filtration system at a temperaturebelow the cloud point of the solutes and generating a solution ofconcentrated organic solutes in water for recycling for use in forwardosmosis processing.

At least some of the solutes can comprise organic solutes. At least someof the solutes can comprise polymeric solutes. At least some of theorganic solutes can have a cloud point well below the boiling point ofwater in the temperature range of about 35-70° C.

Inducing clouding can be achieved by the addition of water-insolublegases in order to lower the cloud point of the solution to ambienttemperatures or thereabout. At least some of the solutes can have amolecular weight of between about 300-800 Daltons. The solutes cancomprise a hydrophobic component and a hydrophilic component to generatesufficiently high osmotic pressures to drive water across asemi-permeable membrane by the process of forward osmosis. The solutescan have a high solubility in the solvent greater than 3 molar ratios togenerate sufficiently high osmotic pressures to drive water across asemi-permeable membrane by the process of forward osmosis.

In various embodiments, systems for purification of a solvent areprovided. A system can comprise a forward osmosis apparatus comprising asemi-permeable membrane, the apparatus configured to permit the creationof an osmotic pressure differential across the membrane using a drawsolution comprising one or more solutes having a cloud pointsubstantially below the boiling point of the solvent, whereby the drawsolution becomes more diluted as solvent is drawn across the membrane.The system can comprise a mechanism for inducing clouding of the solutesin the draw solution after becoming diluted in the forward osmosisapparatus. The system can comprise a filtration mechanism for separatingthe solutes from the solvent in a manner such that the solutes may berecovered for recycling in the forward osmosis apparatus.

For purposes of summarizing the embodiments and the advantages achievedover the prior art, certain items and advantages are described herein.Of course, it is to be understood that not necessarily all such items oradvantages may be achieved in accordance with any particular embodiment.Thus, for example, those skilled in the art will recognize that theinventions may be embodied or carried out in a manner that achieves oroptimizes one advantage or group of advantages as taught or suggestedherein without necessarily achieving other advantages as may be taughtor suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

A general architecture that implements the various features of thedisclosed systems and methods will now be described with reference tothe drawings. The drawings and the associated descriptions are providedto illustrate embodiments and not to limit the scope of the disclosure.

FIG. 1 is a schematic representation of a forward osmosis system.

FIG. 2 is a schematic representation of a fluid purifying systemcomprising a forward osmosis system and a filtration system.

FIG. 3 is a graph correlating draw solution concentration and osmoticpressure.

Throughout the drawings, reference numbers are re-used to indicatecorrespondence between referenced elements. In addition, the first digitof each reference number indicates the figure in which the element firstappears.

DETAILED DESCRIPTION

For a more detailed understanding of the disclosure, reference is firstmade to FIG. 1, which illustrates a schematic representation of anexample forward osmosis system 10. The forward osmosis system 10comprises a feed solution 18 and a draw solution 24 separated by asemi-permeable membrane 30.

The feed solution reservoir 12 is configured to hold a feed solution 18.The feed solution reservoir 12 can receive the feed solution 18 from afirst source (not shown) through a first conduit 16. In the exampleembodiment, the feed solution 18 is in fluid communication with themembrane 30 via an optional first bulkhead 36.

The draw solution reservoir 14 is configured to hold a draw solution 24.The draw solution reservoir 14 can receive the draw solution 14 from asecond source (not shown) through a second conduit 22. In the exampleembodiment, the draw solution 24 is in fluid communication with themembrane 30 via an optional second bulkhead 36.

Feed Solution

The feed solution 18 comprises a fluid to be purified and one or moreimpurities. In at least one embodiment, the feed solution comprisesbetween about 35,000 and 40,000 ppm of salts in water, for example,seawater. In at least one embodiment, the feed solution comprisesbetween about 10,000 and 35,000 ppm of salts in water, for example,brackish water. Many temperatures and pressures are suitable forproviding the feed solution 18. For example, the feed solution 18 can beprovided at ambient temperatures and pressures of seawater.

The systems and methods disclosed herein are also applicable to numerousfluid purification applications where it is desirous to remove at leastone contaminant or solute from a fluid, e.g., waste water purificationand contaminated water purification.

Draw Solution

The draw solution 24 comprises the fluid to be purified and one or morecloud point solutes. A cloud point solute exhibits reverse solubilityversus temperature behavior than what is ordinarily expected for asolute mixed with a solvent. Many temperatures and pressures aresuitable for providing the draw solution 24. For example, the drawsolution 24 can be provided at ambient temperatures and pressures ofseawater.

Cloud Point Solutes

A cloud point solute is soluble in a solvent at lower temperatures but“clouds out” of the solvent as a cloudy precipitate at highertemperatures. Depending on the specific gravity of the cloud pointsolute, either the cloud point solute settles down to the bottom orfloats on top of the solvent. Thus, certain cloud point solutions canexhibit phase separation into two or more immiscible phases at highertemperatures. Preferably, the clouding effect is also reversible. Thatis, upon cooling, the solution will re-clarify.

The temperature at which the dissolved solutes begin to precipitate iscalled the “cloud point.” This behavior is characteristic of certainpolymeric and/or organic compounds, including various surfactants,dispersants, foaming agents, emulsifiers, emollients, and lubricants.

A number of cloud point solute compositions are suitable for use invarious embodiments disclosed herein. Preferably, the cloud pointsolutes have a molecular mass of at least about 200 Da. In preferredembodiments, the cloud point solutes have a molecular mass between about300 Da and 800 Da, e.g., 300, 400, 600 Da, or 800 Da. Preferably, theone or more cloud point solutes are selected such that the solutesexhibit high solubility and/or miscibility in water below the cloudpoint, thus enabling the generation of high osmotic pressuredifferentials, resulting in high fluid flux rates. In certainembodiments, the solubility of the cloud point solutes is at least about2 molar ratios, preferably at least 3 molar ratios to generatesufficiently high osmotic pressures to drive water across thesemi-permeable membrane by the process of forward osmosis.

Certain embodiments include the realization that forward osmosis fluxrates can be improved through the use of cloud point solutes comprisinga hydrophobic component and a hydrophilic component, such as asurfactant. The solubility in water of solutes with a hydrophobiccomponent and a hydrophilic component is based on the hydration of theoxygen groups in the compound via hydrogen bonding. Since this hydrationdecreases with increasing temperature, the solubility of these compoundsin water decreases accordingly. In certain embodiments, these solutesare nonionic organic compounds. In certain embodiments, the solutes arepolymers.

Additional examples of desirable solutes comprising a hydrophobiccomponent and a hydrophilic component are organic compounds withpolyoxyorganic chains, such as polyoxyethylene or polyoxypropylenechains. Preferred solutes include polyethylene glycols (for example,fatty acid or fatty alcohol polyethylene glycols, like FA PEG-300, -400,and -600 and PEG-400 ML) and polypropylene glycols. Additional examplesdesirable solutes include ethoxylates, such as fatty acid ethoxylatesand fatty alcohol ethoxylates.

The cloud point temperature depends on the composition and concentrationof the cloud point solute(s) used. In certain preferred embodiments, thecloud point solute(s) are selected such that the draw solution has asingle cloud point. In various embodiments, the cloud point solutecomposition and concentration are selected such that the cloud point isat least about 10° C. (50° F.) higher than the ambient temperature ofthe feed solution. In certain embodiments, the cloud point is betweenabout 30° C. (90° F.) and 75° C. (170° F.), between about 35° C. (95°F.) and 70° C. (160° F.), or between about 40° C. (100° F.) and 50° C.(120° F.). Preferably, the cloud point solutes have a tendency to cloudat a temperature range from about 45° C. (110° F.) or 50° C. (120° F.)to 70° C. (160° F.) It is preferred that the cloud point solutes have atendency to cloud at a concentration range between about 5% and 20%solute in water.

The stability of various cloud point solvents was tested at varioustemperatures. For temperatures ranging from 25° C. to 100° C., therewere no significant changes found in pH, color, volume, and weight.

The cloud point solutes in the draw solution 24 are at a greaterconcentration than the impurities in the feed solution 18. Preferably,the initial concentration of cloud point solutes in the draw solution isat least two or three times greater than the impurities in the feedsolution. More preferably, the initial concentration in the drawsolution is at least four or five times greater than the feed solution.Most preferably, the initial concentration of the draw solution is atleast eight, nine, ten, or eleven times greater than the feed solution.In certain embodiments, the initial concentration of the cloud pointsolutes in the fluid to be purified is between about 30% and 75%. Incertain embodiments, the initial concentration of the cloud pointsolutes is between about 50% and 70%, e.g., 67.5%.

Membrane

Returning again to the example embodiment of FIG. 1, the membrane 30 isdisposed between two gaskets (first gasket 32 a and second gasket 32 b)and two flanges (first flange 34 a and second flange 34 b). In theexample embodiment, the gaskets 32 a, 32 b and flanges 34 a, 34 b serveto position the membrane in use. The flange-and-gasket system is merelyan example, and alternative techniques can be used for positioning themembrane 30 in the forward osmosis system 10.

The membrane 30 is permeable to the fluid to be purified andsubstantially impermeable to the one or more impurities in the feedsolution. Preferably, the membrane is substantially impermeable to theone or more cloud point solutes in the draw solution.

Preferred embodiments comprise at least one forward osmosis membrane.Examples of suitable forward osmosis membranes include membranesmanufactured by Hydration Technologies Inc. such as SeaPack™, X-Pack™,and Hydrowell™ membranes. The SeaPack™ and X-Pack™ membranes comprisecellulose triacetate (CTA) on a heat-welded polyethylene/polyesterbacking. The Hydrowell™ membrane comprises a CTA cast with an embeddedpolyester screen.

Commercially available reverse osmosis membrane are also suitable foruse in the forward osmosis system 10. These membranes include Thin FilmComposite (TFC) and CTA membranes. TFC membranes are composed ofmultiple layers and use an active thin-film layer of Polyimide layeredwith Polysulfone as a porous support layer. CTA membranes aremanufactured from Cellulose and Acetate. They are organic by nature andrequire disinfection to prevent growth of bacteria. Examples of suitablereverse osmosis membranes are listed below in Table 1.

TABLE 1 Commercially Available Membranes Membrane SpecificationsCulligan Compatible CTA-16- 16 gals/day C RO membrane CulliganCompatible TFM-18a 18 gals/day membrane; reduces RO membrane Fl, Pb, Ba,Cd, Cr, Hg, nitrate, nitrite Hydrotech CTA 15 GPD membrane 15 gals/dayHydrotech TFC 15 GPD membrane 15 gals/day Microline CTA-14 RO membrane14 gals/day Microline TFC-25 GPD RO 25 gals/day; “dry” design, canmembrane be stored for over a yr; replace every 2 yrs RainsoftUltrefiner Compatible 10 gals/day; best for CTA RS9-10 chlorinated waterRainsoft Ultrefiner Compatible 12 gals/day TFC-RS9-12

The specifications for the above reverse osmosis membranes are listedbelow in Table 2. Performance of these membranes, per manufacturers'specifications, is rated by selectivity, chemical resistance,operational pressure differential, and the pure water flow rate per unitarea. The preferred conditions for use of these membranes include timelyhydration of the membranes just prior to use.

TABLE 2 TFC Membranes vs. CTA Membranes Characteristics TFC MembranesCTA Membranes Water rate (to atmosphere @ 22 gals/day 24-35 gals/day 60psi, 77° F.) Maximum feed water TDS 1200 mg/l 2000 mg/l (w/sufficientline pressure) Maximum feed water hardness 10 gpg 10 gpg Rejection ofTDS 88-93% 95-98% Feed water temperature 40-86° F./ 40-113° F./ 4-30° C.4-45° C. Feed water pH 3.0-9.0 2.0-11.0 Maximum feed pressure 50-100 psi50-100 psi (in suitable pressure vessel) Booster pump pressure 80 psi 80psi Chlorine Tolerance excess of sensitive to 10,000 mgl chlorine &other oxidizers

Forward Osmosis Process

The driving force for purification of the fluid is the difference inconcentrations (that is, the difference in osmotic pressures or the“osmotic pressure gradient”) across the semi-permeable membrane 30.Osmosis is the diffusion of a solvent through a semi-permeable membrane,from a solution of lower solute concentration (hypotonic solution) to asolution with higher solute concentration (hypertonic solution). Becauseosmotic pressure is a colligative property, osmosis generally depends onsolute concentration, but not on solute identity. Thus, forward osmosisof a solvent fluid will proceed across a semi-permeable membrane so longas the concentration of one solute is greater than the concentration ofanother solute, regardless of the identities of the solutes.

As the fluid to be purified flows across the semi-permeable membrane 30and into the draw solution 24, the feed solution 18 becomes moreconcentrated over time. When it reaches a certain concentration, thefeed solution 18 can be directed through a first conduit 42 for furtherprocessing or for return to the environment (e.g., as brine). As anexample, a 3.5% NaCl seawater solution can be concentrated to a 10-15%brine solution, prior to discharge back to the ocean. Of course, otherconcentrations levels are suitable for deciding when to remove theconcentrated feed solution 18. In certain embodiments, continuousremoval of the feed solution is contemplated.

Conversely, the draw solution 24 becomes more diluted over time. Thediluted draw solution 24 (or “permeate”) comprising water and one ormore cloud point solutes can be directed through a second conduit 44 forfurther processing. In certain embodiments, the diluted draw solution 24is removed for processing when it reaches a solute concentration ofbetween about 1 and 25%, preferably 10% or less, more preferably about5% or less, and most preferably about 1%. In certain embodiments,continuous removal of the draw solution is contemplated.

Fluid Purification System

Reference is next made to FIG. 2, which illustrates a schematicrepresentation of a fluid purifying system 100 comprising a forwardosmosis system 10 and a filtration system 60. As explained above, theforward osmosis system 10 is configured to accept a feed solution inflow16 and to generate a more concentrated feed solution outflow 42 fordisposal or further processing. The forward osmosis system 10 is furtherconfigured to accept a draw solution inflow 22 and to generate a moredilute draw solution outflow 44, containing the fluid to be purified andcloud point solutes, for further processing.

In certain embodiments, the more dilute draw solution outflow 44 flowsout of the forward osmosis system 10 and into a precipitation system 50.In certain embodiments, the draw solution (not shown) is precipitatedwhile it is in the forward osmosis system 10 (e.g., in the draw solutionreservoir, not shown).

In certain embodiments, the precipitation system 50 comprises a heatingunit. In the heating unit (or in the draw solution reservoir), thetemperature of the draw solution outflow 44 (or draw solution) is raiseduntil the cloud point is reached and, preferably, slightly exceeded. Forexample, the draw solution outflow 44 can be raised until thetemperature exceeds the cloud point by about 2-5° C. When the cloudpoint is reached, the cloud point solute begins to form a cloudyprecipitate, due to lowered solubility in the fluid to be purified (thatis, the solvent). In certain embodiments, a phase separation of the drawsolution or draw solution outflow 44 can be observed. For example, thedraw solution outflow 44 may separate into two immiscible layers, asolvent layer and a clouded solute layer.

In certain embodiments, the precipitation system 50 comprises a gastreatment unit. In the gas treatment unit (or in the draw solutionreservoir), clouding of the draw solution outflow 44 (or draw solution)is achieved by the addition of at least one water-insoluble gas in orderto lower the cloud point of the solution to ambient temperatures orthereabout. For example, methyl chloride can be bubbled through the drawsolution outflow 44. In certain embodiments, the precipitation system 50can comprise a gas treatment unit for lowering the cloud point of thesolution and a heating unit for raising the temperature of the drawsolution or draw solution outflow 44. Other techniques for precipitatinga cloud point solute from solution are also suitable.

After phase separation and/or precipitation has occurred, the drawsolution or draw solution outflow 44 is discharged as a precipitatedsolution 52. The precipitated solution 52 can then be directed to afiltration system 60.

The filtration system 60 is configured to remove the precipitatedcloud-point solute(s) from the precipitated solution 52. In preferredembodiments, the filtration system 60 comprises a nano-filtrationmembrane. An example of a suitable nano-filtration membrane is KochSelRo MPF-34. Alternative filtration systems are contemplated.Furthermore, other membranes such as reverse osmosis membranes and ultrafiltration membranes and combinations of membranes are suitable.

In certain embodiments, the filtration system 60 is selected based on amolecular weight cut-off value (MWCO). In those embodiments, thefiltration system 60 will preferably reject at least 90% of soluteshaving a molecular weight at or above the MWCO. Accordingly, themolecular weight cut-off point (MWCO) of the filtration system 60 shouldbe lower than the molecular weight of the cloud-point solute(s) insolution. Preferably, the MWCO is about 400 Da or less, more preferably,about 300 Da or less, and most preferably 200 Da or less. In certainembodiments, the MWCO is about 100 Da or less.

Certain embodiments comprise a pressurized filtration process, such asnano-filtration or reverse osmosis. Consequently, a pump system 56(e.g., a medium-pressure pump system) can be used to direct theprecipitated solution 52 to the filtration system 60 atsupra-atmospheric pressures. In certain embodiments, the filtrationsystem 60 is operated at a pressure between about 6 and 14 atm (90 and200 psi), more preferably between about 7 and 10 atm (100 and 150 psi),and most preferably between about 8 and 8.5 atm (120 and 125 psi). Forexample, the filtration system can comprises one or more reverse osmosismembranes, and the pump system 56 can direct the precipitated solution56 to the filtration system at 8-14 atm (120-200 psi).

The filtration system 60 discharges a filtered effluent 62 comprisingthe fluid to be purified (e.g., water). In certain embodiments, thefiltered effluent 62 can be directly output as the purified fluid 68(e.g., potable water). In certain embodiments, the filtered effluent 62can optionally be directed through one or more treatment processes 64,such as an activated charcoal filter. Such treatment processes 64 canadvantageously be used to remove any remaining cloud-point solutes,additional organics, and/or microorganisms, etc., from the filteredeffluent 62. After passing through the one or more optional treatmentprocesses 64, the filtered effluent 62 can be output as the purifiedfluid 68. Preferably, the total dissolved solids (TDS) levels thatremain after processing are between about 5 and 50 ppm.

Optionally, in certain embodiments, the filtration system 60additionally discharges a re-concentrated draw solution 22. One exampletechnique for preparing the re-concentrated draw solution 22 is flushingthe filtration system 60 with a supply of fluid 72 at a temperaturebelow the cloud point of the solutes. In an example embodiment, there-concentrated draw solution 22 is prepared by flushing or rinsing ananofiltration membrane in the filtration system 60 with cold water. Thefluid 72 redissolves precipitated cloud-point solutes collected in thefiltration system 60.

The re-concentrated draw solution 22 can then be returned to the forwardosmosis system 10. Such a configuration advantageously enables a closedcycle for the cloud-point solutes to be reused in the forward osmosisfluid purification process.

U.S. Pat. No. 5,679,254 to Chakraborti discusses desalination ofseawater by nonionic surfactant aided phase separation. The '254 patentdescribes a method for recovering relatively salt-free water by addingnonionic surfactants to aqueous salt solutions. The surfactants used arefrom a family of nonionic compounds called alkoxylates, which exhibitmultiple cloud points to create various biphasic solutions. Thetemperature of the mixture is raised above a first cloud point to causethe mixture to phase separate into a first predominantly salt-rich phaseand a second salt-free phase, the salt-free phase comprising water andnon-ionic surfactant. The salt-free phase is subsequently separated fromthe salt-rich phase. The temperature of the separated salt-free phase isfurther raised above the second cloud point to promote an additionalphase separation into a surfactant-rich phase and a surfactant-freephase. The surfactant-free phase is then separated to provide potablewater.

The '254 patent differs from the systems and methods disclosed herein.For example, the '254 patent relies upon a nonionic surfactant whichpossesses multiple cloud points. In contrast, the disclosed systems andmethods can function with a draw solution having a single cloud point.As another example, the '254 patent requires co-solvents likecyclohexanol and 2-ethylbutyl cellulose for the final water separationfrom the surfactant. In contrast, the cloud point solutes disclosedherein can be directly separated from the fluid to be purified using afiltration unit, such as a nanofiltration or ultrafiltration membrane,without requiring additional chemicals to be added.

Table 3 below shows the energy requirements for different conventionaldesalination processes used by various public water utilities. The datain Table 3 demonstrate that the systems and methods disclosed herein canreduce the operating costs of desalinating water by a factor of three tofive over existing desalination technologies, such that the costs arecomparable to wholesale freshwater sources.

TABLE 3 Energy Requirements for Desalination Processes Energy costs,Process kWh/acre-ft of water Reverse Osmosis  6,000-12,000 Multi-stageflash distillation 3,500-7,000 Multi-effect distillation 2,500-5,000Vapor Compression 10,000-15,000 Forward Osmosis (estimated for4,000-6,000 Osmotics Technologies Inc. process) Forward Osmosis (asdisclosed) 1,500-3,000

EXAMPLE 1 Determination of Cloud Point

The cloud point temperatures of cloud point solutes were determined attemperatures ranging from 10 to 70° C. and concentrations varying from10 to 90%. Visual measurements were made at 10° C. intervals. Forexample, the clarity/cloudiness of the individual solutions wasassessed. The data collection for one example of the cloud point solutestested, a fatty acid-polyethylene glycol (FA-PEG) called PEG 400 ML, isshown in Table 4 below. In Table 4, “θ” represents a visibly clearsolution; a single asterisk “*” represents very slight clouding; adouble asterisk “**” represents more pronounced clouding; and a tripleasterisk “***” represents substantial clouding. A correlation betweenconcentration, temperatures, and cloud points can be noted, enabling adetermination of the operational design points of the forward osmosissystem at varying solute concentrations and temperature.

TABLE 4 Organic solute PEG 400 ML in water/temperature experiments Temp.Water Dilution (%) (° C.) 10 20 30 40 50 60 70 80 90 10 θ θ θ θ θ θ θ θθ 20 θ θ θ θ θ θ θ θ θ 30 θ θ θ θ θ θ * * ** 40 θ θ θ θ θ θ * ** ** 50 θθ θ θ θ * *** *** *** 60 θ θ θ θ * *** *** *** *** 70 θ * * ** *** ****** *** *** 80 90

No cloudiness was observed below 25° C., which is higher than the inletseawater temperature for most areas of North America. At 30° C., slightcloudiness for solutions containing more than 70% water was observed.Complete cloudiness and phase separation occurred above 50° C. for the70-90% water solution.

Based upon Table 4, an example process can be derived to treat aseawater feed solution having a temperature of 20° C. A concentrateddraw solution with 50% concentration of PEG-400 ML is prepared. The drawsolution is diluted through forward osmosis as fresh water moves fromthe feed solution through a semi-permeable membrane, until the drawsolution reaches about 90% water content (that is, a 10% concentrationof PEG-400 ML). The draw solution is subsequently heated to about 50° C.to trigger clouding of the PEG-400 ML. The clouded draw solution(comprising precipitated PEG-400 ML and water) is passed through ananofiltration membrane to remove the precipitated PEG-400 ML. Whererecycling is included, the PEG-400 ML collected on the nanofiltrationmembrane is dissolved back into solution as a re-concentrated drawsolution by flushing the nanofiltration membrane with cold water tocreate a 50% concentrated draw solution.

Numerous other processes for obtaining purified fluids from an impurefeed solution using cloud point solutes can be derived as well followingsimilar principles, for example, using cloud point solutes with varyingranges of cloud points over different concentrations and temperatures.

EXAMPLE 2 Cloud Point Solute Concentration

Higher concentrations of cloud point solutes in the draw solution 24compared with the concentration of impurities in the feed solution 18can advantageously improve recovery rates of the fluid to be purified.FIG. 3 shows osmotic pressures and water recovery rates obtained for anexample forward osmosis system, from a seawater solution having anosmotic pressure of 27.2 atm. In this example, ammonium carbonate wasused as the solute in the draw solution. However, as described in moredetail above, because osmotic pressure depends on the number ofdissolved particles in solution, but not on the identities of thesolutes, similar results would also be obtained using cloud pointsolutes. As shown in this example, a 50% recovery of water from seawatercan be achieved by using a draw solution having an osmotic pressure thatis greater than twice that of seawater, while a 75% water recovery canbe achieved with a draw solution having an osmotic pressure that isgreater than four times that of seawater. As shown here, the researchteam was able to demonstrate a 90% recovery of freshwater using forwardosmosis through a semi-permeable membrane. As shown in FIG. 3, theconcentration of the draw solution is directly proportional to theosmotic pressure generated.

EXAMPLE 3 Forward Osmosis Membranes

Suitable membranes were evaluated for use with the forward osmosisprocesses disclosed herein. Prior to use, all membranes were soaked indeionized water for at least 15 minutes to allow any glycerin or sodiummetabisulfite to diffuse out of the membrane. Each membrane testedagainst selected draw solutions. The draw solutions were prepared indifferent weight percentages to give rise to different osmotic pressuresas shown in Table 5, below, which lists certain example initial osmoticpressures for FA-PEG cloud point solutions. The membranes and drawsolutions were placed in the forward osmosis system 10 shown in FIG. 1,with a feed solution concentration of 3.5% NaCl in water.

TABLE 5 Osmotic Pressures for Various Solutes in Water Solute (%)Solvent Osmotic Pressure (atm) Seawater (3.5% NaCl) water 27.2 FA-PEG300 (50%) water 79.9 FA-PEG 300 (70%) water ~113.9 FA-PEG 400 (50%)water ~150 FA-PEG 400 (75%) water ~237.3 FA-PEG 600 (67.5%) water ~305

Each solution was tested against several membranes, the feed solutionand draw solution were monitored over a period of 4 to 5 hours, and thedata were collected. At least three trials were carried out for eachdraw solution and membrane. From the collected data, flow rates werecalculated and compared. The calculated flow rates for reverse osmosismembranes are shown in Tables 6a and 6b, below. The calculated flowrates for forward osmosis membranes are shown in Table 7, below.

TABLE 6a Flow Rates, in mL/hr per cm² of membrane (by trial, membraneand draw solution) Culligan (RO) Hydrotech (RO) CTA (no backing) Trial 

1 2 3 1 2 3 1 2 3 50% FA-PEG 300 0.062 0.062 0.062 0.062  0.062 0.062 xx x 70% FA-PEG 300 0.12 0.12 0.062 0.062 0.12 0.062 x x x 50% FA-PEG 4000.12 0.062 0.062 0.062 0.12 x x x x 75% FA-PEG 400 x 0.062 0.062 0.062 x0.062 x x x 67.5% FA-PEG 600 0.062 0.062 x 0.062 0.12 0.062 x x x x -denotes no movement in draw and feed level

TABLE 6b Flow Rates, in mL/hr per cm² of membrane Microline (RO)Rainsoft (RO) Trial 1 2 3 1 2 3 50% FA-PEG 0.062 0.062 0.062 0.062 0.0620.062 300 70% FA-PEG 0.062 0.062 0.062 0.062 0.12 0.12 300 50% FA-PEG0.12 0.12  x 0.062 0.12 0.062 400 75% FA-PEG 0.062 X 0.062 0.062 0.0620.062 400 67.5% FA-PEG 0.062 0.062 0.062 0.12 0.062 0.062 600

TABLE 7 Flow Rates, in mL/hr per cm² of membrane (by trial, membrane anddraw solution) Sea-Pack (FO) X-Pack (FO) Hydrowell (FO) Trial 

1 2 3 1 2 3 1 2 3 50% FA-PEG 300 0.19 0.25 0.14 0.17 0.17 0.22 0.28 0.360.17 70% FA-PEG 300 0.24 0.25 0.2 0.21 0.31 0.29 0.36 0.37 0.31 50%FA-PEG 400 0.11 0.25 0.20 0.16 0.17 0.19 0.26 0.23 0.29 75% FA-PEG 4000.24 0.25 0.23 0.21 0.23 0.23 0.28 0.25 0.36 67.5% FA-PEG 600 0.21 0.230.19 0.21 0.22 0.2 0.28 0.28 0.28

In these tests, it was observed that membranes specifically manufacturedfor forward osmosis resulted in higher flow rates than membranesspecifically manufactured for reverse osmosis. The flow rates of theforward osmosis membranes were almost 2 to 3 times faster than flowrates for commercially available reverse osmosis membranes.

EXAMPLE 3 Draw Solution Testing

Forty combinations of draw solutions and membranes were evaluated, asshown below in Table 8. Eight different membranes, both reverse osmosismembranes and forward osmosis membranes, were evaluated. Five differentdraw solutions were evaluated. Seawater was used as a feed solution ineach system. The seawater was changed daily until the draw solution nolonger increased in its volume.

TABLE 8 Flow Rates (mL/hr per cm² of membrane) Culligan HydrotechMicroline Rainsoft CTA No HTI HTI HTI Sea- CTA CTA CTA CTA BackingHydrowell X-Pack Pack FA-PEG 0.062 0.062 0.062 0.062 0 0.320 0.170 0.165300 50% FA-PEG 0.12 0.062 0.062 0.12 0 0.365 0.300 0.245 300 70% FA-PEG0.062 0.091 0.120 0.062 0 0.275 0.165 0.225 400 50% FA-PEG 0.062 0.0620.062 0.062 0 0.265 0.230 0.245 400 75% FA-PEG 0.062 0.062 0.062 0.062 00.280 0.215 0.220 600 67.5%

Although each draw solution initially had a different osmotic pressure,the point where the draw solution no longer drew water molecules acrossthe membrane was roughly the same. For example, 20 mL of a 50% solutionof PEG 300 had a 10 mL increase in volume, and 20 mL of a 50% solutionof PEG 400 also increased the same amount in volume. As shown in Table8, the highest flow rates were observed with a PEG 300 cloud pointsolute at 70% concentration in water using HydroWell™ membranes.

Although this invention has been described in terms of certain preferredembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments which do not provide all thebenefits and features set forth herein, are also within the scope ofthis invention.

1. A forward osmosis fluid purification system comprising: asemi-permeable membrane having a feed solution-facing surface opposite adraw solution-facing surface; a feed solution in communication with thefeed solution-facing surface of the semi-permeable membrane, wherein thefeed solution comprises a fluid to be purified and impurities dissolvedin the fluid to be purified; a draw solution in communication with thedraw solution-facing surface of the semi-permeable membrane, wherein thedraw solution comprises at least one cloud point solute, and wherein theconcentration of the at least one cloud point solute in the drawsolution is greater than the concentration of the impurities in the feedsolution; and a precipitation system configured precipitate the at leastone cloud point solute from the draw solution.
 2. The system of claim 1,wherein the feed solution comprises seawater or brackish water, andwherein the fluid to be purified is water.
 3. The system of claim 1,wherein the solubility of the at least one cloud point solute in thefluid to be purified is a molar ratio of at least 3:1.
 4. The system ofclaim 1, wherein at least one cloud point solute has a molecular weightbetween about 300 Da and 800 Da.
 5. The system of claim 1, wherein atleast one cloud point solute comprises a hydrophobic component and ahydrophilic component.
 6. The system of claim 4, wherein at least onecloud point solute comprises a polyoxyorganic chain.
 7. The system ofclaim 4, wherein at least one cloud point solute comprises apolyethylene glycol or a polypropylene glycol.
 8. The system of claim 7,wherein at least one cloud point solute comprises a polyethylene glycolselected from the group consisting of FA PEG-300, FA PEG-400, PEG-400ML, and FA PEG-600.
 9. The system of claim 5, wherein at least one cloudpoint solute comprises an ethoxylate.
 10. The system of claim 9, whereinat least one cloud point solute comprises a fatty acid ethoxylate or afatty alcohol ethoxylate.
 11. The system of claim 1, wherein theconcentration of the at least one cloud point solute in the drawsolution is between about 30% and 75%.
 12. The system of claim 11,wherein the concentration of the at least one cloud point solute in thedraw solution is between about 50% and 70%.
 13. The system of claim 1,wherein the precipitation system comprises a heater configured to heatthe draw solution to at least the cloud point temperature of the drawsolution.
 14. The system of claim 1, wherein the precipitation systemcomprises a gas treatment system configured to add at least onewater-insoluble gas to the draw solution to lower the cloud point of thedraw solution.
 15. The system of claim 1, further comprising afiltration system configured to remove the precipitated at least onecloud point solute from the fluid to be purified in the draw solution.16. The system of claim 15, wherein the filtration system comprises ananofiltration membrane.
 17. The system of claim 15, wherein thefiltration system comprises an ultrafiltration membrane.
 18. The systemof claim 15, further comprising a redissolution system configured toredissolve the precipitated cloud point solutes to form a recycled drawsolution.